A large peak-to-average power ratio reduces the efficiency of a transmission power amplifier. In particular this is a problem for multi-carrier multiplexed signals such as OCDM and OFDM signals as for such signals the maximum possible peak power is much larger than the average transmitted power.
U.S. Pat. No. 6,175,551 shows a method for reduction of the peak-to-average power ratio by means of soft-clipping. Soft-clipping means that a scaled reference function is subtracted from the sampled signal, such that each subtracted reference function reduces the peak power of at least one signal sample. Preferably the reference function has approximately or exactly the same bandwidth as the transmitted signal. In this way it is assured that the peak power reduction does not cause any out-off band interference. One example of a suitable reference signal is a sinc function.
A common disadvantage of prior art soft-clipping methods is that the subtraction of the reference function from the over shooting signal portion can result in multiple maxima. This problem of the prior art is illustrated by making references to FIGS. 1 to 3:
FIG. 1 shows the magnitude signal 100 of an I,Q signal to be transmitted. Signal 100 has portions which are above clipping threshold 102. Those portions of signal 100 respective peaks are detected and marked by peak markers 104 to 114.
FIG. 2 shows reference signal 116 in the time domain. Scaled reference signal 116 is subtracted from the I and Q components of signal 100 at the peak locations which are identified by peak markers 104 to 114. The result of this soft-clipping operation is shown in FIG. 3 where magnitude signal 118 is the clipped signal.
For example, the first peak of signal 100 which is identified by peak marker 104 is transformed into two new peaks 120 and 122 of signal 118. Likewise a large number of new peaks is created in the signal portion consisting of the peaks identified by peak markers 108 to 114. Hence additional soft-clipping iterations are required to interatively bring signal 100 to clipping threshold 102.
Therefore the present invention aims to provide an improved method of reducing a peak-to-average power ratio of a signal to be transmitted for improved soft-clipping.